Kim Jong-un and his younger sister Kim Yo-jong in an undated picture released in March 2015.
Photograph: KCNA/EPA

The sister of the North Korean leader, Kim Jong-un, will attend the opening ceremony of the Winter Olympics in South Korea on Friday, becoming the first member of Kim’s immediate family to cross the border.

Seoul’s unification ministry said it had been notified by Pyongyang of Kim Yo-jong’s landmark trip. She will be accompanied by Kim Yong-nam, North Korea’s nominal head of state, Choe Hwi, the head of the national sports guidance committee, and Ri Son-gwon, who led last week’s inter-Korean talks.

The ministry said the inclusion of Kim Yo-jong in the delegation was “meaningful” because she is a senior official in the ruling Workers’ party.

The Olympics have triggered a rapid rapprochement on the peninsula after tensions soared last year when North Korea carried out multiple weapons tests. Seoul has advocated for increased engagement with the regime, possibly in the hopes that the Olympic detente can be continued after the Games, putting it at odds with the US, which is determined to take a hard line.

On Wednesday, during a visit to Japan, the US vice-president, Mike Pence, said the “toughest and most aggressive” economic sanctions against North Korea would be announced shortly.

Pence, who will also attend Friday’s opening ceremony in Pyeongchang, described the Kim regime as the world’s “most tyrannical and oppressive”. Washington would not let North Korean propaganda “hijack” the Olympics, he said.

Kim Yo-jong’s trip could provide a source of contention between Seoul and Washington, as she was blacklisted last year by the US treasury department over human rights abuses and censorship. Choe is subject to a travel ban under UN security council sanctions.

Kim Yo-jong is the vice-director of the North Korean propaganda and agitation department, which handles ideological messaging through the media, arts and culture. Choe previously worked for the same organisation.

Meet Kim Yo-jong, the sister who is the brains behind Kim Jong-un's image

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Having previously appeared only occasionally and in the background at public events, Kim Yo-jong has taken a more prominent role recently, assisting her brother at numerous state ceremonies.

On Tuesday, she was pictured in the state media greeting a North Korean art troupe that has since departed for the South to perform during the Olympics.

Prof Koh Yu-Hwan, of Dongguk University, said Pence’s planned presence in Pyeongchang may have been a factor in Kim’s decision to send his sister. “Her trip will create a media frenzy, stealing the show from Vice-president Pence who wants to highlight the North’s human rights abuses and sanctions-defying nuclear drive,” he told AFP.

Shin Beom-chul, a professor at the Korea National Diplomatic Academy in Seoul, said: “One of the positives of her visit is that she is someone able to deliver a direct message on behalf of Kim Jong-un. What is problematic is that she’s coming with Choe Hwi. This raises worries that North Korea likely intends to use this Olympics as a propaganda tool rather than a possible opening to meaningful dialogue with South Korea.”

A group of 280 North Koreans arrived in South Korea on Wednesday, one of the largest peacetime crossings of the Korean border, to help spur on athletes from the two countries at the Winter Olympics, which start on Friday.

The delegation, made up mostly of a 229-member cheer squad, reached a border checkpoint by bus at about 9.30am local time. In addition to the cheering squad, there were 26 taekwondo performers, 21 journalists and four North Korean Olympics committee members, including the sports minister, Kim Il-guk.

After going through security controls, the group left for the Olympics venue in Pyeongchang. On Tuesday, a North Korean ferry crossed the border carrying a 140-person orchestra to perform during the Games.